The present invention generally relates to motor control and particularly relates to speed and positioning control of brushless servomotors.
Brushless servomotors use electronic commutation of phase currents rather than electromechanical (brushes) commutation. Such motors generally have a permanent magnet rotor and a wound stator, and commutation is a function of rotor position.
Commutation refers to actively steering currents or voltages to the proper motor phases at the proper times, and is done electromechanically in brush motors via brushes and a commutator. In brushless motors, however, switching electronics perform motor commutation using rotor position information as indicated by a motor feedback signal. Such feedback signals usually are generated by Hall effect sensors responsive to the motor magnets, or by sensing motor-induced voltages, or by sensing encoder pulses, etc.
A number of disadvantages arise in such contexts. For example, the speed control resolution of Hall sensor feedback typically is poor. Generally, the angular resolution and accuracy for Hall sensor feedback is no better than the pole/magnet count of the motor and the positioning accuracy of the Hall sensors. Further, speed control based on sensing induced voltages, while offering potential cost advantages over Hall sensor based control, typically works only at higher motor speeds where the induced signals are robust enough for reliable control feedback.
Such disadvantages represent particularly significant challenges in certain motor applications. For example, brushless dc motors provide an economical and reliable means for driving the various printing subassemblies used in a variety of print systems, such as laser printers, photocopiers, etc. However, relatively precise motor rotation control is required in such systems to maintain acceptable printing quality over a potentially wide range of printing process speeds.